The present invention refers to a method and an apparatus for curtain coating a moving support with a liquid coating material where a lateral flow is added to the guided curtain on both sides, the width of the curtain being greater than the width of the coating on the support, and the lateral flow being supplied transversely to the length of the curtain and in parallel to the front wall of the lateral guides. The invention further refers to a device for carrying out the method, comprising two lateral guides for the curtain and an installation allowing the supply of said liquid film in parallel to the rear wall of the lateral guides, as well as a device for the extraction of the liquid film. Such a method and apparatus are described in EP-A-0 740 197. Although the planar guides disclosed therein provided a certain progress over the prior art of the time, they still have drawbacks. Thus, from the point where the curtain is formed, the falling velocity of the curtain continuously increases in the falling direction due to gravitation, whereas in the liquid film which runs down along the lateral guide is not affected by gravitation because the surface velocity of the liquid film assumes a constant value on account of internal friction. When the curtain meets the lateral guide, it is only possible to match the two velocities at a single point, and a reduced stability of the curtain results due to local distortions and constrictions. The coating solution may be any kind of coating solution which is formed as a liquid coating material.
One of the main problems of curtain coating is the conservation of a stable curtain, particularly in the vicinity of the lateral guides which are necessary in order to prevent contraction of the curtain on account of surface tension forces.
The geometric design of the lateral guides of the curtain is particularly problematic if the width of the curtain is smaller than that of the coated support.
Basically, another form of lateral curtain guides according to EP-B-0 414 721 may be used, which describes a linear apparatus where the lateral guide consists of a straight bar. The disadvantage of linear lateral guides is that the falling curve of the liquid curtain depends on different parameters such as its viscosity, surface tension, volume flow, the geometric design of the pouring lip where the curtain is formed, the direction of the initial velocity of the curtain relative to the direction of the gravitation vector, etc. In most cases, the falling curve of the curtain is not linear, and it varies from one application to another if one of the above parameter varies.
Although in principle, planar lateral guides according to the patent application cited in the introduction do not have this drawback, they have other disadvantages as described. EP-A-0 737 521 also describes an approximately planar lateral guide, but the wetting liquid is supplied from above in this case as well, and a device for separating the edge from the center portion of the curtain is required.